This work involves a study of the physiological and anatomical properties of nerve muscle junctions developing in vivo. The system being investigated is the regenerating arm of the adult newt. At the appropriate stages of limb developing, one can find muscle fibers which are uninnervated. Slightly more mature fibers show several intermediate phases of synapse maturation. (1) At the earliest stage spontaneous synaptic potentials occur but stimulation of the motor nerve trunk does not evoke synchronous transmitter release. (2) At other junctions maximal nerve stimulation evokes only a single end-plate potential of low quantum content. (3) More mature fibers receive synaptic input from as many as four motor neurons, which can be distinguished by their discrete stimulus thresholds. During this period of synapse development the fibers lack an action potential but often show a prolonged response to depolarization. Investigations are currently underway to determine the anatomical correlates and the physiological explanations for the developmental phases encountered. Also the membrane properties of developing muscle fibers are being examined.